Introduction

When a PCB is tested in an in-circuit test fixture, a high density of probes can be in contact with the test card simultaneously. The widespread usage of 0.050" and 0.039" probes in high density areas leads to significant flexion of the unit under test (UUT). For example, a one inch square BGA can require 400 probes at 0.050" spacing. Consider 8oz probes, this component will support 200 lbs/in2 underneath its body.

In the case of a single-sided fixture, all the probes are applied underneath the card and a vacuum box secures the card from the top.

Ideally, due to the forces on each side of the UUT, all probes should be counterbalanced by a pressure finger. If this were possible, the resulting force applied to the card would been nil. Unfortunately, this is rarely possible. The density and complexity of electronic circuits is increasing constantly. We therefore find ourselves constructing fixtures with a high density of probes that cannot be supported adequately, resulting in major flexion of the card under vacuum.

As we will see, major flexion of a UUT can result in breakage of certain components and enormous repair costs for damaged circuits. Moreover, some problems such as a solder fracture may not be detected during final test, or even worse, after delivery to the end client. It is therefore essential to use the best tools available to minimize this effect when designing our test fixtures.

Stress, strain and displacement

For a thin plate to bend, as in the image below, the top of the plate must stretch, while the bottom must be compressed. The stretching and compression of this plate depend on the displacement created by force P.

Strain:	ε = ΔL / L
Stress:	σ = E * ε = P / A L = Distance E = Young's modulus P = Force A = Surface (impact distance)

Strain is measured in millionths of a unit. For example, 500 microstrains represents a strain of:

500 microstrains:

= 500 X 10E-6 in / in = 0.0005 in / in

Thus, 500 microstrains represent a strain of 0.0005" over a distance of one inch. The components installed on a PCB suffer the same strains. Typically, through-hole components are not very sensitive to strain because of their flexible lugs.  However, large surface mount components such as BGAs are very sensitive to strain.

Certain manufacturing processes such as Electroless Nickel/Immersion Gold Plating, widely used for installation of BGAs, render the solder joints very brittle under certain conditions. Since they have a rigid body and are installed directly on the PCB surface, these components withstand stretching to such an extent that they will suffer solder fracture failures. It has also been proved (1) that the increasingly widespread use of lead-free soldering renders the solder joint even more brittle. The Bansal, Yoon & Mahadev study (2) shows that the most exposed solder joints are located at the edges of the components. Large components soldered to a card, such as BGAs, will have the effect of reinforcing the PCB at this location. The stresses exerted on them will then be distributed to the weakest locations, the outer edges.

Strain gages

Strain gages are small devices used to measure surface strain. Perfectly adhering to a PCB’s surface, the sensor will stretch according to the PCB surface strain at a precise location.

These devices are made from a very thin metallic element with electrical resistance that varies according to stretching or compression. As in Image 3, a sensor installed at point A will stretch and indicate a positive value (e.g.: 600 microstrains). However, a sensor installed at point B will contract and indicate a negative value (e.g.: -600 microstrains). The strain gages are unidirectional; i.e. show strain on one axis. Certain superimposed models, as in Image 4, can show strain on several axes simultaneously. When used appropriately, the strain gage is a very precise and indispensable tool.

When installed inside a fixture during a vacuum test, strain gages let us obtain a real-time reading of the strain involved on the PCB. The Strain vs. Time Graph (graph above) that we provide with each stress analysis represents the strain over a period of 10 seconds. During this period, vacuum is applied to and removed from the fixture. The red, yellow and blue curves in the above graph represent the three different axes of the sensors used. The black curve represents the maximum diagonal strain calculated according to the following equation:

MAX( |e1+e2-e3|, |e3| )

Typically, we recommend that the maximum strain be less than 600 microstrains at all times. Strain levels are directly proportional to vacuum levels. All our tests are performed at a maximum vacuum level, between 28 and 29 inches of mercury.

Strain gages can be installed at any place where high strains are expected from probe concentrations. An IPC guideline for strain gage testing (3) recommends that all BGA components with a body size equal to or larger than 1 inch should be tested using strain gages. One strain gage should be installed on all four corners of the component. If all the BGAs on the PCB are smaller than 1 inch, it is recommended to test the three largest devices.

Strain rates

The strain rate is the increase in strain per second (microstrain/sec). The Bansal, Yoon & Mahadev study (2) showed the relationship between BGA failures and strain rates. The faster a force is applied to the PCB, the less force is required to damage the UUT. A weak force applied very quickly does more damage than a strong force applied slowly. The study shows that it is possible to repeat  BGA  fractures in production during high-speed flexion tests (>5000 microstrain/sec). At low speeds (about 500 microstrain/sec), they showed that some components even withstood up to 6800 microstrain. However, at high speeds, a strain of 1000 microstrains was enough to damage the components. We suggest a maximum strain of 600 microstrains at all times.

In the above graph, the points beyond |5000| should be considered as an indication to a zone where the strain should be less than 600 microstrains. Between the seventh and eighth second, we can see that the strain decelerates to a speed of about -6200 microstrain/sec. In the next graph, we can confirm that these strain values are under the established limit.

It is thus essential to measure and control the strain rates. The speed at which the probe and vacuum forces are applied to the UUT can be easily controlled by using a vacuum regulator or using more or less supportive springs between the fixture's base and support plates. Only appropriate equipment using strain gages can measure strain rates.

Software Analysis

Finite Element Analysis (FEA) is a numerical technique for finding solutions of boundary-value problems in structural analysis. FEA is commonly used for determining stresses and displacements in mechanical systems. FEA software is applied in various engineering environments and is generally adapted to popular CAD packages for use in diversified applications. The highly complex and versatile algorithms of this software can take several hours to solve a system including thousands of components, such as the probes in our in circuit test fixtures.

Here at Rematek, we have chosen to use an alternate method called Finite Difference Simulation (FDS). This numerical technique is perfectly adapted to our problems involving a thin plate and provides us with fast and precise results. The FDS analysis tool is directly integrated into our fixture design software called DrillCAD. This software, which has been continuously developed since 1994, has turned out to be the most sophisticated fixture design tool on the market. Its brand-new FDS functions now allow us to predict displacements, pressures and strains everywhere on the PCB very precisely during the fixture design stage.

Our FDS tool not only lets us account for the force of the probes, the positioning of the fingers/stoppers and the thickness of the plate, but also the presence of components on top of and underneath the PCB. The 2D analysis software usually used in the fixturing industry considers the PCB to be a plate with uniform elasticity. Such software will compute that the maximum strain under a BGA is located at its centre. In practice, we know that the forces are concentrated on the outer edges of a BGA.

DrillCAD has been programmed according to a plate with non-uniform properties. It is thus possible for us to define the zones where the components add rigidity to the UUT. As described in the example in Image 3, a BGA will concentrate the forces and strains on its edges, reducing these same factors at its centre.

The following examples are showing the displacement and strain as a colored map over the UUT image, with and without the component effect. The red and magenta zones represent displacement greater than 0.005" or strain over |500|.

Perfectly adapted to analysis of a thin plate such as a PCB, our FDS tool lets us obtain results with multiple iterations in a few minutes. Thus, we can quickly optimize the design of our fixtures to minimize the strain levels on the PCBs, without lengthening the delivery times of our projects. Once this work is completed, DrillCAD allows us to export all the necessary files to manufacture the fixture, such as the drilling files, the wiring files and the test files.

Better fixturing solutions

To reduce the pressures exerted on a PCB, the first and simplest solution is to reduce the probes' forces whenever possible. Secondly, the positioning of the fingers/stoppers must be optimized to control the probe forces, but this is often very difficult to achieve. Mechanically, the stoppers must be located exactly under the pressure fingers to avoid the creation of shear points (Image 5).

The density of the components on both sides of the PCBs often makes it impossible to obey this rule. It is then necessary to eliminate the fingers/stoppers by using contact surfaces that will maximize the support points.

In  addition  to  supporting  the  PCB  perfectly  by  eliminating  a  large  part  of  the deformations, the Push Plate method (Image 6) has the advantage of significantly improving the precision of the assembly. The probes are guided to their contact point with no possibility of deviation. However, it is necessary to clear all the components to avoid any possibility of contact between the pressure plate and the components. Since component positioning tolerances vary from one manufacturer to another, some conflicts can still arise between components and the pressure plate, even though free space was cleared in the cavities. To minimize this possibility, we therefore use a combination of the two methods.

This Pressure Islands method (Image 7) represents the perfect compromise between the standard method and pressure plates. The PCB is perfectly supported thanks to pressure islands in the high-deformation zones identified with our FDS tool and supported with fingers/stoppers where there is enough space. This method has shown excellent results.

Summary

Strain gage measurement is a reliable method of finding and resolving excessive strains on a PCB, and should be implemented in the fixture preventive maintenance routine. Strain gage tests must be conducted after every modification that can affect the strain in the fixture. Such modifications include probe changes and pressure finger or stopper modifications.

BGA fractures can be induced not only in in-circuit test fixtures, but also during board assembly, testing, system integration, packaging and shipping. We suggest consulting the IPC JEDEC-9704 document (3) for specific guidelines on strain gage testing.

Our FDS simulation tool, used jointly with our strain analysis equipment, has allowed us to develop new and very efficient fixturing methods. Significantly reducing the deformation zones on PCBs and potential fractures in the production environment, our methods are constantly evolving to always meet our clients' needs.

For more information, please feel free to contact our Customer Service Department at This email address is being protected from spambots. You need JavaScript enabled to view it..

Author

Marco Deblois
President Rematek Inc.
Montreal, Quebec, Canada

References

1. PREDICTING BRITTLE FRACTURE FAILURES by P. Borgesen, B. Sykes, Surface Mount Technology, October 2005

2. FLEXURAL STRENGHT OF BGA SOLDER JOINTS WITH ENIG SUBSTRATE FINISH USING 4-POINT BEND TEST by A. Bansal, S. Yoon, V. Mahadev, Altera Corporation, January 2005

3. IPC/JEDEC-9704, PRINTED WIRING BOARD STRAIN GAGE TEST GUIDELINE, Developed by the JEDEC Reliability Test Methods for Packaged Devices Committee (JC-14.1) and the SMT Attachment Reliability Test Methods Task Group (6-10d) of the Product Reliability Committee (6-10) of IPC, June 2005